1. Field of the Invention
This invention relates to a lightweight paper comprising a coaggregate formed from agglomerated particles of urea-formaldehyde polymer and agglomerated particles of hydrated silicic acid, and a process for producing the same.
2. Description of Related Art
Making printing paper and newspaper roll, lightweight has been conducted by making paper thinner, but there may occur an obstacle of the so-called print through. Such print through includes show through due to reduction in the opacity of paper (hereinafter referred to as "opacity") and the so-called strike through that printed ink runs through paper and it is seen from its back surface (hereinafter the extent to which it is prevented will be referred to "printing opacity"). Either of the above physical properties refer to a phenomenon that print on the back surface is seen from the front surface to make reading of print on the front surface difficult; hence such physical properties are most important at the time of making lightweight paper.
Agglomerated particles of hydrated silicic acid are called whilte carbon and have been used for preventing the print through (e.g. U.S. Pat. Nos. 4,132,806, 4,161,455, 4,157,920 and 4,202,813) but they have almost no effect of improving the opacity, and moreover, the percentage fixation thereof relative to pulp at the time of paper-making is so weak that the retention thereof is low.
Further, agglomerated particles of urea-formaldehyde polymer have been used for improving the brightness of paper and the opacity (e.g. see U.S. Pat. No. 3,909,348), but the effect of improving the printing opacity is still insufficient although the agglomerates of urea-formaldehyde polymer has a stronger fixability onto pulp than that of white carbon and hence the percentage retention is higher.